Base it off of an existing 5e race
Since the race does not exist in 5e, you will need to homebrew it. In general, the best place to start creating a "new" race in 5e is to base it off of an existing race. The Dungeon Master's Guide has some rules and guidelines for doing this around page 285, "Creating New Character Options - Creating a Race or Subrace". Specifically, it says
When creating a race from scratch, begin with the story and proceed from there. Compare your creation to other races of your world, and borrow freely from the traits of other races.
It also gives an example of how they changed Tieflings into Aasimar using the guidelines.
For your purpose, I would suggest basing it off of the Fairy race, limiting the "Fey Characteristics" section to having wings (instead of bird-like, make it furry wings). Maintain the flight aspect, have them speak Common and Undercommon, change the lifespan to be 200-300 years, etc.
The hardest part would be changing the spells Fairies innately know. According to the Forgotten Realms Wiki, Gloamings were good with shadow-based spells. In general, you should replace spells with spells of similar level that do similar things (i.e. if it does damage, the new one should also do damage, etc.), or you can opt to keep the same spells.
Here is an example stat block I would possibly recommend:
- Ability Score Increase. Increase one ability score by 2, and increase a different one by 1, or increase three different scores by 1.
- Languages. You can speak, read, and write Common and Undercommon.
- Creature Type. You are a Fey.
- Life Span. Gloaming have a life span of 200-300 years.
- Size. You are Small.
- Speed. Your walking speed is 30 feet.
- Gloaming's Shadow Magic. You know the Prestidigitation cantrip. Starting at 3rd level, you can cast the Cause Fear spell with this trait. Starting at 5th level, you can also cast the Blindness/Deafness spell with this trait. Once you cast Cause Fear or Blindness/Deafness with this trait, you can't cast that spell with it again until you finish a long rest. You can also cast either of those spells using any spell slots you have of the appropriate level. Intelligence, Wisdom, or Charisma is your spellcasting ability for these spells when you cast them with this trait (choose when you select this race).
- Flight. Because of your wings, you have a flying speed equal to your walking speed. You can't use this flying speed if you're wearing medium or heavy armor.
Using Detect Balance (a google docs spreadsheet commonly used to help with homebrew balance), this is certainly still on the strong side (although so is the Fairy). You can add some things like light sensitivity if it would help with balance in your game. At the end of the day, though, whatever stat block you or your GM come up with will require some playtesting to really figure out if it is properly balanced and - most importantly - fun.
Disclaimer: I am only knowledgeable in 5e, not 3e. The information presented here is a combination of my 5e knowledge and what I found in the Forgotten Realms Wiki. This stat block is not tested for balance, and is not meant to treat, diagnose, or prevent any illness.